1. Field of the Invention
The present invention relates to a functional device having functional elements which perform processing, such as conversion of input signals, alteration of a path, selection of a wavelength and enabling/disabling electrical connection, method of manufacturing for the functional device, and a driver circuit, more particularly, to a functional device which controls the operations of functional elements by a microelectromechanical section.
2. Description of the Related Art
For optical connection network systems, such as an optical fiber system of a wavelength division multiplexing (WDM) type, there are growing needs for the technique that switches optical paths and the technique that selects light of a predetermined wavelength from input light. Such an optical connection network system uses an optical switch at each node on a network, which selectively demultiplexes light of a predetermined wavelength from light with a plurality of wavelengths and then changes the path of the light. The probable future increase in the amount of communication information to be transferred requires the multi-channel and large-scale design of optical devices, such as an optical switch.
Because an optical switch classified into the optical devices changes the path for light without photoelectric conversion of the light, it has characteristics, such as the possible minimizing of the delay time, no dependence on the transfer speed and the expandability. Conventional methods of providing optical switches, which have been proposed so far, include a method which employs the mechanical motion of an optical fiber, a method which is based on the Faraday rotation and a method which uses a reflecting mirror.
Because optical switch which uses a reflecting mirror and employs a microelectromechanical system (MEMS) for the reflecting mirror and a drive apparatus which drives the reflecting mirror is manufactured by using the microfabrication technology to fabricate semiconductor integrated circuits, it is advantageous in cost reduction and large-scale fabrication and is expected as an optical switch which can sufficiently meet the need for the larger scale fabrication of optical switches which will be originated from the future multi-channel design.
For example, Japanese Patent Laid-Open No. 2000-314846 discloses a reflecting mirror formed by an MEMS. Specifically, Japanese Patent Laid-Open No. 2000-314846 discloses a technique of providing a reflecting mirror coupled to a supporting block in a rotatable manner by a beam portion, attaching an electrode to the supporting block and applying a voltage to the electrode so that the operation of the reflecting mirror is controlled by electrostatic force generated between the electrode and the reflecting mirror. Japanese Patent Laid-Open No. 2001-117025 discloses a reflecting mirror formed by an MEMS too. Further, Japanese Patent Laid-Open No. 330254/1999 discloses a technique such that in a semiconductor device equipped with switch means which has a plurality of MOS transistors formed on a substrate and a plurality of switch elements or MEMS formed on the MOS transistors, the switch elements perform switching by moving interconnections provided movably using the Coulomb force. Japanese Patent Laid-Open No. 330254/1999 also describes that with that technique, a variable logic LSI having a higher freedom of design can be realized by providing invariable connection in a semiconductor device with the MOS transistors and providing variable connection with the switch elements.
Japanese Patent Laid-Open No. 144596/1999 discloses a technique of forming an RF switch using an MEMS on a semiconductor monolithic microwave integrated circuit substrate. This technique provides a beam supported rotatably in a seesaw form on the substrate and applying a voltage to an electrode arranged near the beam, thereby generating electrostatic force between the beam and the electrode, which turns the beam. This allows a terminal formed on the substrate to have contact or no contact to a terminal formed on the bottom side of the beam, thereby opening or closing the switch. Japanese Patent Laid-Open No. 144596/1999 describes that the use of the technique can form an array of RF switches with a good sensitivity.
U.S. Pat. No. 5,963,788 (Carole C. Barron, et al.) discloses a technique of preparing a driver circuit which drives MEMS elements on the same silicon substrate on which the MEMS elements are formed.
Japanese Patent Laid-Open No. 2002-36200 discloses a technique of integrating MEMS device modules and an IC control circuit module needed to drive the MEMS device modules on a common systems connecting substrate. This can ensure easy separation of the MEMS device modules and the IC control circuit module for replacement or repair.
The above-described prior arts however have the following problems. While electrostatic force, magnetic force, a piezoelectric effect, thermal expansion and so forth are available as the drive force for a functional element, such as a reflecting mirror or RF switch, a device equipped with such a functional element needs a driver circuit to generate such drive force. In case where electrostatic force is used as the drive force for a reflecting mirror, for example, a driver circuit which selects and controls an MEMS to be driven is needed in addition to an applied voltage generating circuit which generates a voltage.
As shown in FIG. 1 of the aforementioned Japanese Patent Laid-Open No. 2001-117025, for example, such a conventional driver circuit is prepared on a substrate separate from a substrate on which a functional element, such as a reflecting mirror, and a drive apparatus for driving the functional element (hereinafter generally called xe2x80x9cMEMS elementxe2x80x9d) are formed, and is connected to the substrate on which the MEMS element is formed by wire bonding or a flexible substrate or the like. If the scale of an optical device becomes larger due to the multi-channel design and the number of MEMS elements to be driven is increased, the number of interconnections to connect the driver circuit to the individual MEMS elements and the scale of the driver circuit increase, resulting in the enlargement of the overall apparatus. That is, although driving and controlling MEMS elements require electrodes, the number of terminals for exchanging drive control signals with an external unit is increased due to the multi-channel design and the large-scaling of the array, thereby increasing the area needed to lay out the associated interconnections. If two electrodes are needed to drive a single MEMS element, for example, a total of 2n2 electrodes are needed for the square matrix layout (array) of n rows by n columns (n being an integer), and terminals equal in number to 2n2 should be provided on the device, which would result in a larger area needed to layout the interconnections to connect to those terminals.
According to the technique disclosed in U.S. Pat. No. 5,963,788, a cavity portion is provided on the top surface of a silicon substrate and an MEMS element is formed in the cavity portion after which a driver circuit is formed in an area in the top surface of the silicon substrate, which is different from the cavity portion. The technique therefore requires a step of protecting the MEMS element at the time of forming the driver circuit and a planarization step after the formation of the driver circuit. This results in an undesirable increase in the number of required steps. In case of laying out an array of several thousand light reflecting mirrors formed by MEMS elements in order to achieve the multi-channel design, the ratio of the area of the cavity portions occupying on the top surface of the silicon substrate increases, thus reducing the mechanical strength of the silicon substrate at the time of preparing the apparatus.
Further, as the technique disclosed in Japanese Patent Laid-Open No. 2002-36200 arranges a plurality of MEMS modules on the systems connecting substrate in a replaceable manner, it is necessary to secure the inter-module alignment precision. In case where each MEMS module includes a mirror for use in optical communications, for example, the optical paths among the modules should be secured precisely. This would result in a complicated assembling process or lower the reliability of assembled apparatuses. Because each MEMS module takes a sealed structure itself, it has a larger volume than an MEMS chip, which makes the apparatus larger in the large-scale array design.
Accordingly, it is an object of the invention to provide a functional device which can suppress a size enlargement originated from an increase in the number of interconnections that is caused by a multi-channel design, is easy to fabricate, has a higher mechanical strength and contributes to cost reduction and an improvement on the reliability, a method of manufacturing the functional device and a driver circuit mounted on the functional device.
A functional device according to the invention comprises a plurality of functional elements each of which processes an input signal and outputs that processed signal; a driver circuit substrate having a substrate and a driver circuit, provided on the substrate, for driving the functional elements; a joining layer having an insulating layer, formed of an insulating material, for joining the functional elements to the driver circuit substrate and connection terminals, provided in the insulating layer, for connecting the functional elements to the driver circuit.
According to the invention, the functional elements and the driver circuit substrate which has the driver circuit for driving the functional elements are provided and the functional elements are connected to the driver circuit substrate by the joining layer. Even in case where a large number of functional elements are provided so that the functional device takes a multi-channel design, therefore, it is possible to suppress the enlargement of the overall functional device which is originated from an increase in the area of the driver circuit.
As the distances between the driver circuit and the functional elements can be made shorter, the interconnections therebetween can be made as short as possible. Further, because the driver circuit is formed on the driver circuit substrate and the functional elements are joined to the driver circuit substrate by the joining layer, the fabrication is easier, the strength of the substrate does not become lower, and there is less integration and physical misalignment between the driver circuit substrate and the functional elements, thus making it possible to improve the reliability of the functional device and reduce the production cost, as compared with the case where functional elements are formed directly on the substrate.
Furthermore, the joining of the functional elements to the driver circuit substrate by the joining layer can improve the reliability of the functional device and miniaturize the device, as compared with the case where replaceable modules are used. Because the functional elements and the driver circuit substrate can be fabricated independently, it is possible to independently determine whether the functional elements and the driver circuit substrate are satisfactory or not. This can lead to an improvement of the overall yield of functional devices.
It is preferable that the functional device should further comprise input/output terminals for connecting the driver circuit to an external circuit and the number of the input/output terminals should be less than the number of the connection terminals. This can reduce the number of interconnections between the driver circuit and an external circuit, making it possible to decrease the area of the portion that is needed for the layout of the interconnections.
Each of the functional elements may have a processing element for processing the input signal; a microelectromechanical section for supporting the processing element in a movable manner; and a drive electrode for moving the processing element by generating electrostatic force between the drive electrode and the processing element to which a voltage from the driver circuit is applied. This can allow an electrical signal output from the driver circuit to be converted to the mechanical operation of the processing element with a simplified structure.
In this case, each of the functional elements may have at least three drive electrodes. This design can allow the processing element to be moved freely, thus increasing the degree of freedom of signal processing.
The signal may be an optical signal, the processing element may be a light reflecting mirror for reflecting at least part of the optical signal, the microelectromechanical section supports the light reflecting mirror in manner rotatable, and the functional device may perform optical switching as the drive electrode controls an angle of the light reflecting mirror and the light reflecting mirror selectively outputs the optical signal input.
In this case, the drive electrode may be comprised of a transparent conductor, the light reflecting mirror may be semitransmissive, the substrate may be formed of a transparent insulator and the driver circuit substrate may have a photodetecting substrate including a photodetecting element on that side which does not face the functional elements. This can allow an optical signal to be always monitored at the time of performing optical communications using this optical device. As a result, it is possible to detect an abnormality of an optical signal which passes the functional device and the disconnection or the like of the communication path.
Alternatively, the signal may be an optical signal, the processing element may be a filter for selectively separating light of an arbitrary wavelength from the optical signal, the microelectromechanical section may support the filter in a reciprocatable manner, and as the drive electrode of the functional device controls a position of the filter to intervene the filter in a pass band of the optical signal, the filter may selectively separate light of an arbitrary wavelength from the optical signal input and output the separated light.
Alternatively, the signal may be an electrical signal, the processing element may be a switch member which, when deformed, connects an input terminal to which the electrical signal is input to an output terminal, and the drive electrode may deform the switch member to select enabling or disabling of supply of the electrical signal to the output terminal.
The driver circuit may have an array of transistors; a single gate line or a plurality of gate lines connected to gate electrodes of the transistors; a plurality of drain/source lines connected to source electrodes of the transistors; terminals, connected to drain electrodes of the transistors and the drive electrodes, for applying voltages applied to the drain electrodes to the drive electrodes; and a drain/source driver circuit for selectively inputting a signal to the drain/source lines. Even if an optical switch takes a multi-channel design and has a large scale, therefore, the circuit portion is not enlarged, thus further suppressing the enlargement of the functional device.
Another functional device according to the invention comprises a functional-element movably supporting structure having a functional element for performing optical processing on at least part of light input to a surface of the functional element and outputting the processed light and a microelectromechanical section for supporting the functional element and controlling an operation of the functional element; and a driver-circuit-substrate structure arranged on that side where the functional element is not provided as seen from the functional-element movably supporting structure and having a substrate of an insulator and a driver circuit, formed on the substrate, for controlling an operation of the microelectromechanical section.
According to the invention, the functional-element movably supporting structure having the functional element and the microelectromechanical section is provided and the driver-circuit-substrate structure is arranged on that side where the functional element is not provided as seen from the functional-element movably supporting structure. Even in case where the functional device takes a multi-channel design, therefore, it is possible to suppress the enlargement of the overall functional device which is originated from an increase in the area of the driver circuit.
As the driver circuit is formed on the substrate of an insulator in the driver-circuit-substrate structure, there is less integration and physical misalignment between the driver-circuit-substrate structure and the functional-element movably supporting structure, thus making it possible to improve the reliability of the functional device and reduce the production cost. Further, because the distance between the driver circuit and the microelectromechanical section can be made shorter, the interconnections therebetween can be made as short as possible, thereby ensuring the miniaturization of the functional device and an improvement on the reliability thereof.
A method of manufacturing for a functional device according to the invention comprises the steps of: forming a processing element and a microelectromechanical section for supporting the processing element the processing element in a movable manner on a silicon substrate; forming a through hole in an insulating substrate; forming a first electrode on a first side of the insulating substrate and a second electrode on a second side of the insulating substrate, the second electrode being connected to the first electrode via the through hole; preparing a functional-element movably supporting structure by joining the processing element and the microelectromechanical section to the insulating substrate; preparing a driver circuit substrate by forming on a substrate a driver circuit which drives a functional element; and joining the functional-element movably supporting structure to the driver circuit substrate in such a way that the driver circuit is connected to the second electrode.
A driver circuit according to the invention, which is provided in a functional device having a plurality of functional elements for each processing an input signal and outputting the processed signal and drives the functional elements, comprises an array of transistors; a single gate line or a plurality of gate lines connected to gate electrodes of the transistors; a plurality of drain/source lines connected to source electrodes of the transistor; terminals, connected to drain electrodes of the transistors and the functional elements, for applying voltages applied to the drain electrodes to the functional elements; and a drain/source driver circuit for selectively inputting a signal to the drain/source lines.
As apparent from the above, the invention can provide a functional device which, even if taking a multi-channel design, can suppress the enlargement of the functional device and ensure cost reduction and an improvement on the optical communications.